Cathode ray tube and method of operation



June 9, 1964 R. T. WATSON 3,136,918

CATHODE RAY TUBE AND METHOD OF OPERATION Filed Dec. 16, 1960 mmvron.Fazszr 7. MUM

This invention relates to improvements in cathode ray tubes of the typeemploying post-deflection acceleration of the electron beam and tomethods of operation thereof.

One type of cathode ray tube includes an envelope having a faceplate, aneck, and an interconnecting funnel section. An electron gun is disposedin the neck and projects an electron beam toward the faceplate. Means isprovided either as a part of the electron gun or as a separate meansexternal of the envelope for bidirectionally deflecting the electronbeam as it passes through a primary deflection zone located in theregion where the neck is I joined to the funnel.

It has recently been proposed to provide such tubes with post-deflectionacceleration arrangements wherein a mesh electrode is mounted across theenvelope in the electron beam path just beyond the primary deflectionzone. In such an arrangement, the practice has been to operate the meshelectrode at a potential equal or approximately equal to the potentialon the last electrode of the gun and to operate a conductive coatingelectrode on the funnel and on the screen at a considerably higherpotential. Thus, the electrons of the beam are projected at a relativelylow velocity through the primary deflection zone, and then, afterpassing through the mesh electrode, are greatly accelerated for highvelocity impingement on the phosphor screen.

In such tubes according to the prior art, the mesh electrode has eitherbeen mounted as a part of the electron .gun'or as a separate electrodemounted in the funnel section of the envelope. Although gun-mounting ofthe mesh electrode is desired for simplicity of tube fabrication, funnelmounting arrangements are, on the other hand, preferred for beamdeflection reasons and for wide deflection angle tubes. Mounting of themesh as a part of -the gun limits the transverse dimension of the meshelecnode to one which can be inserted through the neck of the tube.Thus, deflection in the primary deflection zone is physically limitednot by the diameter of the neck opening, but rather by the diameter ofsome frame structure of the mesh electrode.

On the other hand, when the mesh electrode is mounted in the funnel thebeam can be deflected a greater. amount in the primary deflection zonepermitting extremely wide-angle tubes to be de signed In extremelywide-angle deflection tubes of the type described, it is'desirable thatmaximum deflection in the primary deflection zone be obtained. Thisdesirability requires that the mesh electrode thus. be mounted in thefunnel of the envelope which results in a relatively extensive spacingof the mesh from the electron gun and an extremely close spacingof themesh from the phosphor screen. As a result, secondary electrons emittedby the -mesh are attracted to the screen rather than to a gun electrodeand result in decreased contrast of the produced image.

In cathode ray tubes of the type described, it is also known to correctcertain raster distortions such as pin- United States Patent U M3,136,918 Patented June 9, 1964 cushioning by selectively shaping themesh electrode, e.g., into various domed contours. However, for purposesof economy, it is preferred to avoid complex mesh electrode contours. I

It is therefore an object of my invention to provide a new and improvedcathode ray tube of the type described and a method of operation thereofwhich provides good scan sensitivity, improved contrast of the image,and economical correction of common raster distortions.

Briefly, according to my invention, a cathode ray tube of the typedescribed employs a novel combination of post-deflection accelerationand post-deflection deceleration of the electron beam. Three separateconductive surfaces are provided on the inner wall of the envelope suchas by coatings of conductive material. A first coating extends from themesh electrode back into the neck of the tube. A second coating isdisposed on the funnel and spaced from both the mesh electrode and thephosphor to each of the conductive coatings.

In the operation of the cathode ray tube, according to my invention, agiven voltage is applied tothe first coating; a substantiallyhighervoltage is applied to the second coating; and a voltage having avalue between that applied to the first and second coatings is appliedto the third coating. Accordingly, a post-acceleration system isprovided wherein the electron beam is deflected at a relatively lowvelocity to provide scan sensitivity, and then upon passing through themesh electrode is rapidly accelerated. Following this, the electron beamis decelerated before impinging upon the phosphor screen. By virtue ofthe decelerating field wherein the phosphor screen voltage is lower thanthe voltage on the funnel, secondary electrons emitted from. the meshelectrode are attracted to the funnel, thus improving image contrast.

According to a preferred embodiment of my invention, the first andsecond conductive coatings on the neck and funnel, respectively, areseparated by a specially-shaped high resistance coating to provide adesired control of linearity and pincushion correction. An annularconductive plate is mounted at one of its edges to adjacent one edge ofthe high resistance coating and extends out over the high resistancecoating. This plate prevents electrons from impinging on the highresistance coating and thereby prevents an objectionable charge build-upof electrons on the coating.

In the drawings:

FIG. 1 is an elevation view with parts broken away and partly in axialsection of a preferredembodiment of a cathode ray tube according to myinvention; and

FIG. 2 is a transverse section view through the funnel of a cathode raytube according to my invention looking toward the neck, and wherein' themesh electrode and electron gun are removed so as to more clearlyillustrate the shape of the high resistance coating.

Structure In FIG. 1 a cathode ray tube 10, according to my invention, isshown to comprise a glass envelope 12 having a faceplate 14, a neck 16,and an interconnecting funnel section 18. A phosphor screen 20 isprovided on the inner surface of the faceplate 14. The phosphor screen20 may comprise any suitable phosphor material appliedby any of thewell-known techniques such as -settling,-slurry-ing, or evaporating. a

" An electron gun 22 is mounted in the neck 16 and is.

adapted to project an electron beam 23 toward the phosphor screen 20.The electron gun 22 m'ay'be of any suitable type as is also well knownin the art. Suitable electrical potentials are applied to the electrodesof the gun 22 through terminal prongs 24 which form part of a stem basestructure 26. v

Suitable means, such as'magnetic deflection 0011s 28, is

provided for bidirectionally deflecting the electron beam 23 in aprimary deflection zone 32. As is shown, the primary deflection zone 32comprises the region just beyond the electron gun 22 where the neck 16joints the funnel section 18. Y

A dome-shaped mesh electrode'34 is disposed transversely in the path ofthebeam 23 adjacent the primary deflection zone 32 and between the zone32 and the phosphor. screen 20. The mesh electrode 34 comprises amounting ring 36 to which a dome-shaped mesh member is attached. Themounting ring 36 is in turn fixed to the funnel section 18, such asbycementing thereto or by sealing it in or throughthe glass wall of thefunnel 18.

Although a domed shaping of the mesh electrode 34is preferred for reasonhereinafter described, other shapes,-

such as flat, may be provided.

In accordance with myinvention, the

- 1 ward the corners of the'rectangular phospho'r'screen 20. As will be'hereinatfer described, suchf lobed shape of the I resistive coating 50serves to desirably shapethelectrostatic fields which 'afiect theresulting shape of the scanned 1 inner surface of i r the envelope 12 isprovided withthree separated conductive surfaces. In the case of theall-glass envelope 1 2, as I illustrated, the conductive surfaces areprovided as conductive coatings on the glass envelope. G e

A first conductive coating -38 is provided on a first portion of theenvelope extending from and in contact withthe mounting ring 36 towardand into the neck 16.

. The final electrode 40 of, the electron gun 22 may also be .44 may bespecially shaped along its edge adjacent the first conductive coating38. Both the first conductive coating 38 and the secondconductivecoating 44 may comprise any known, suitable material, such as,a carbon I cussed hereinafter in more detail. n t With respect to theabove-mentioned feature 'ofpre screen coatings, respectively; 'However,it

oxide. Provision of the high resistance coating 50 results in auniformfpotential drop fromthe neck coating 7 38 to the funnel coating44, prevents the' area between V the neck and funnel coatings 38 and 44fromretaining the chargefof any electrons which might impinge there on,and also prevents the corners of the scanned raster .from expandingtoorapidlythereby causing pincushioning andnonlinearity. This latterfeature will be dis- .vention of pincushiom'ng, theQresistive coating5.0is

[preferably specially shaped as shown in FIG. 2. As there 'shown inthecase of a tube 10 having ajgenerally rec? tangular phosphor screen20,the resistive coating 50 is shaped to comprise'four lobes 52 extendinggenerally toraster on'the phosphor screen 20.

An annular metallic shield 53 is provided ini'conjunce '7 A ,tion'withthe high resistance coating 50. The shield 53 is mounted at-itsinneredge'to therim36 of the mesh electrode'ancl extendsout over theresistance coating 50; 4 The shield 53 prevents excessive impingement ofelectrons upon the resistance coating 50 whenithe beam 23 is 'scannedclosely thefreto j v a V The three conductive'coatings 38, 44,'and46-are electrically separated from each other-so that they maybecomposition. Such a material Widely used in the industry 1 is known asAquadag.

A third conductive coating is provided over the phosphor screen20. .Thethird coating 46 may comprise a suitable, known material, such asaluminum, evaporated upon the phosphor layer 20.

the phosphor layer 20, an aluminum layer. be evaporated upon the organiclayer, andthen the organiclayer be' volatilized by baking and therebyremoved. .According to such known techniques, a smooth conductive thinaluj minum coating 46 is provided on the phosphor layer 20.

The third conductive coating 46 is coextensive with the phosphor layer20 and is spaced from the second conductive coating 44 by an annulararea 48. The annular area 48 may be coated with a high resistancematerial or, as shown, may comprise only the bare surface of theenvelope 12. Inorder tomount the mesh electrode 34 within the envelope 12, it may bede'sired to provide' access for mountingby providing aseparatefaceplate 14 and funnel 18, which are then frit sealed togetherin the region of the annular area 48; Accordingly, a special highresistance frithaving a conductivity greater than ordinary glass canbe'used so as to provide the same -etfect'as would be achieved bycoating the annular area 48 with a high resistance material, such asiron oxide.

For purposes of simplicityof reference,1the first coat- 'ingf38, secondcoating 44, and third coatingi46 will hereinafter be referred tosimplyas the neck, funnel, and

According to well-known techniques, an organic material may first beappliedto operated at' three different electrical 1 potentials, Ac-

cordingly, separatederminal means 54, .56, and 58 are providedformakingelectrical connections,respectively, 1 to the'neck, funnel, and screencoatings 38, 44,and 46.

As shown inFIG. 1, the terminal means'54, 15,6,'and 58 may compriselead-ins sealed through the 'glass envelope 12 and contactingtheir'associated conductive coatings.

2 Operation; 2 I

. Thetube 10 is operated withanovel combination {of post-deflectionacceleration and post-deflection deceleration of the electron beam23.Post deflection acceleration is obtained by applying a relatively lowpotential .to

' the'neck coating'38' and a relativelymuch higher potential to thefunnel' coating 44. These potentials may for example, be respectivelyina ratio ofapproximately 1 to 2. 'In accordance with such operation,the" electron A 1 "beam 23 is projected at relatively low velocitythrough the primarydefiection zone 32' and is thus'given an exceptionally large deflection by a comparatively low' power magneticdeflection yoke 28. After passing through the mesh electrode 34, therelatively high voltage foni the funnel coating 44 causes theelec'tronbea'm 23 to be substantially accelerated.' Thus, ,both good scansensitivity and go'o'd'lightoutput are obtained. a

Deflection sensitivity is enhanced not only bythe low ivelocity beamdeflection in the primary. deflection zone 32, but also by virtue.of-the electrostatic field established between theme'sh electrode 34and the funnel coating 44.

' This field is generally illustrated by the equipotential lines A, B,C, and D. When the electron beam 23 is projected through theaccelerating field ABCD in a peripheral re-" gion as indicated in FIG;1, the accelerating field ABCD, by'virtueof its shape;v tends'todefiectthe beam 23 fur- -ther:out toward the edge 'of-the phosphorscreen 20' to provide further scan enhancement: I

Prew'ously proposed dome mesh post-accelerating tubes' have comprised asingle conductive coating 'over boththe funnel and the phosphor screenso that these portionsof the tube were necessarily operated at the sameelectrical 1 potential. V In such "tubes, secondary electrons emittedwill be apprei "Iciated that the neck coating'38 may ac'tually extend apart way onto the funnel18 as shown. The neckand 1 funnel coatings 38and 44 are separated by a highre sistance' coating 50 of ajsuitablematerial, suchlas iron from the mesh electrode caused a reduction incontrast in two different ways: (1) secondary electrons emitted from thefront of the mesh electrode, i.e., facing the phosphor screen, wereattracted directly to the phosphor screen, and resulted in diffusedluminescence thereof; (2) secondary electrons emitted from the back ofthe mesh electrode, i.e., facing the electron gun, were focused throughthe mesh electrode openings and attracted onto the phosphor screen togive a halo effect around the beam.

This halo results from the fact that the secondary electrons are lowerin velocity than the primary beam electrons and hence respond to thesupplementary deflection field ABCD to a greater extent than do theprimaries.

In tubes of this type wherein the funnel coating 44 is separate from thescreen coating 46, such reduced contrast is substantially prevented byoperating the screen coating 46 at a potential somewhat below thepotential on the funnel coating 44. I have found that a screen potentialapproximately halfway between the potentials on the neck and funnelcoatings 38 and 44 is suitable.

By applying a lower potential to the screen coating 46 than exists onthe funnel coating 44, a decelerating electrostatic field isestablished. This field is represented by the equipotential lines E, F,G, and H. In the case of an electron beam 23 projected off the centralaxis of the tube 10, such as is illustrated in FIG. 1, the deceleratingfield EFGI-I will serve to supplement the deflection of the electronbeam 23. Even more important though, secondary electrons emitted fromthe mesh electrode 34 will now be attracted to the higher potentialfunnel coating 44 rather than to the phosphor screen 20.

It will be appreciated that the novel combined post acceleration andpost deceleration operation does not necessarily require that the highresistance coating 50 or the annular shield 53 be included in the tube10. The resistive coating 50 and the annular shield 53 constitutepreferred embodiment features. Even in the absence of these features,the three-coating, three potential, post-acceleration-decelerationoperation provides significant improvement over prior art tubes.

Although it has been proposed by the prior art to correct pincushioningby a special shaping of the dome mesh 34, it is preferred that anycomplex fabrication of the mesh electrode 34 be avoided, if possible.According to my invention, the mesh electrode 34 can be provided with asimple spherical curvature. Pincushion correction can then be obtainedby a special shaping of the resistive coating 50. FIG. 2 illustratesthis special shaping. As shown therein, the resistive coating 50 whenused with a generally rectangular phosphor screen 20, comprises fourlobes 52, one each extending generally toward the corner of therectangular phosphor screen 20. The effect of the lobes 52 is to weakenthe concentration of the equipotential lines A, B, C, and D in theregion where they curve in toward the edge of the mesh electrode 34 andthe funnel coating 44. Such weakening of the field results in theelectron beam 23 being subjected to less of a supplementary deflectionas it emerges from the mesh electrode 34. Since this weakening of theelectrostatic field ABCD is provided generally in line with the cornersof the rectangular raster scanned on the phosphor screen 20, the effectis that the corners of the raster are pulled in, thus correctingpincushioning.

As previously stated, the annular shield 53 serves to prevent electronsfrom striking the resistive coating 50 and thus cause an electron chargeto be built up thereon. Such charge build-up would ordinarily occur whenthe beam 23 is scanned very close to the resistive coating 59 duringthat time when the corners of the raster are being scanned. Thus, suchcharge build-up would occur generally in four localities, i.e., in theregion of each of the lobes 52. The result of such charge build-up isthat the beam in passing adjacent the charged areas is deflected awaytherefrom. Thus, a clipping of the corners of the raster results. Suchcorner clipping would not constitute a correction of pincushioning sinceit would in fact amount to merely clipping the corners rather thanshrinking the overall deflection in the directions toward the corners.The presence of the annular shield 53 exerts some effect on the shapingof the electrostatic field ABCD. Should it be desired to utilize to thefullest advantage a specialized shaping of the domed mesh electrode 34,then it may be desired to eliminate the annular shield 53.

According to a preferred embodiment and operation of the tube 10, theresistive coating 50 was shaped as shown in FIG. 2 and the annular metalshield 53 was mounted to the support ring 36 of the mesh electrode 34.The phosphor screen 20 conformed to that of industry standard 21 inchtube types. The mesh electrode 34 was provided with a sphericalcurvature of approximately 1.9 inches radius and the diameter of themounting rim 36 was about 4 inches. Voltages of 10 kv., 22 kv., and 17kv., were applied respectively to the neck, funnel, and screenconductive coatings 38, 44, and 46. The final electrode 40 of theelectron gun 22 was connected to the neck coating 38 by contacts 42 asillustrated. A magnetic deflection yoke of the type generally employedfor providing a maximum angle of deflection, approximately of thatproduced in the tube 10, was used and energized for such 80% deflection.With the tube 10 operated as herein stated, a full raster was obtainedwhich has good linearity, good pincushion correction, and good contrast.

What is claimed is:

l. A cathode ray tube comprising an envelope, including a faceplate, aneck, and an interconnecting funnel, a

phosphor screen on said faceplate, an electron gun in said neck forprojecting an electron beam upon said screen, a mesh electrode mountedacross said envelope in said beam path adjacent the neck end of saidfunnel, a first conductive coating on said envelope extending from saidmesh toward said gun, a second conductive coating on said envelopebetween and spaced from both said screen and said mesh, a thirdconductive coating on said screen, a high resistance coating on saidenvelope separating said first and second conductive coatings, anannular conductive shield supported at one edge thereof from adjacent anedge of said high resistance coating and extending out over said highresistance coating, and separate external terminal means connected toeach of said conductive coatings for supplying different electricalpotentials thereto.

2. A cathode ray tube comprising an envelope including a faceplate, aneck, and an interconnecting funnel, a phosphor screen on saidfaceplate, an electron gun in said neck for projecting an electron beamupon said screen, a mesh electrode mounted across said envelope in saidbeam path adjacent the neck end of said funnel, a first conductivesurface on said envelope extending from said mesh toward said gun, asecond conductive surface on said funnel between and spaced from bothsaid screen and said mesh, a third conductive surface on said screen,and means including a voltage source for applying a first electricalpotential to said first conductive surface, a second electricalpotential substantially higher than said first electrical potential tosaid second conductive surface, and a third electrical potential havinga value between said first and second electrical potentials to saidthird conductive surface.

3. The cathode ray tube according to claim 2 and wherein said secondpotential is appproximately twice said first potential and said thirdpotential is approximately halfway between said first and secondpotentials.

4. The cathode ray tube according to claim 3 and wherein said firstpotential is about 10 kv., said second potential is about 22 kv., andsaid third potential is about 17 kv.

5. A cathode ray tube comprising an envelope including a generallyrectangular faceplate, a neck, and an interconnecting funnel, agenerally rectangular phosphor screen on said faceplate, an electron gunin said neck for projecting an electron beam upon said screen, a meshelectrodemounted'across said envelope in said beam path 'adjacent to theneck end of said'funnel, a first conductive coating on said envelopeextending from said mesh toward said gun, a second conductive coating onsaid er'ivelope between'and spaced from both said screen and said mesh athird conductive coating on said screen, a

high resistance coating on said envelope separating said "first andsecond conductivecoatings and having a shape comprising four lobesextending generally toward the four corners of said rectangular screen,and separate external terminal nieans connected to each of saidconductivecoatings for supplying different electrical potentialsthereto.

References Cited in the file of this patent UNITED STATES PATENTSBurdick etl; Apr. 25, 1961

5. A CATHODE RAY TUBE COMPRISING AN ENVELOPE INCLUDING A GENERALLYRECTANGULAR FACEPLATE, A NECK, AND AN INTERCONNECTING FUNNEL, AGENERALLY RECTANGULAR PHOSPHOR SCREEN ON SAID FACEPLATE, AN ELECTRON GUNIN SAID NECK FOR PROJECTING AN ELECTRON BEAM UPON SAID SCREEN, A MESHELECTRODE MOUNTED ACROSS SAID ENVELOPE IN SAID BEAM PATH ADJACENT TO THENECK END OF SAID FUNNEL, A FIRST CONDUCTIVE COATING ON SAID ENVELOPEEXTENDING FROM SAID MESH TOWARD SAID GUN, A SECOND CONDUCTIVE COATING ONSAID ENVELOPE BETWEEN AND SPACED FROM BOTH SAID SCREEN AND SAID MESH, ATHIRD CONDUCTIVE COATING ON SAID SCREEN, A HIGH RESISTANCE COATING ONSAID ENVELOPE SEPARATING SAID FIRST AND SECOND CONDUCTIVE COATINGS ANDHAVING A SHAPE COMPRISING FOUR LOBES EXTENDING GENERALLY TOWARD THE FOURCORNERS OF SAID RECTANGULAR SCREEN, AND SEPARATE EXTERNAL TERMINAL MEANSCONNECTED TO EACH OF SAID CONDUCTIVE COATINGS FOR SUPPLYING DIFFERENTELECTRICAL POTENTIALS THERETO.